1. Field
The invention is in the field of blast and drill hole sampling.
2. State of Art
During the drilling of blast holes in an open pit mine (or similarly during drilling in connection with mineral deposit exploration) the drill employs a rotary or impact (hammer) technique in penetrating underground from the surface. Material in the drill hole shatters from the drill action into small pieces with particle size typically ranging in a continuous distribution from as large as one-inch size through dust size fines. Compressed air is introduced through the hollow center of the drill pipe to transport the shattered rock (termed cuttings or drill hole cuttings) from the bottom of the hole to the surface. When applied to drilling blast-holes, composition data of material sampled during this process is used for mine production planning. Sampling can be done during removal of cuttings from the drill hole, or, alternatively, from an accumulation of drill cuttings after drilling is completed. Sampling of the cuttings from blast holes has the objective of obtaining a quantity of drilled out material suitably representative of the total volume and particle size distribution of drill cuttings removed. This is necessary so that the composition analysis carried out on the sample will be of appropriate statistical accuracy. With composition data from drill hole samples, planning of production from the mine can be developed to optimize selection of ore material to be processed and waste (non ore) material to be dumped. Efficient execution of mine production is greatly dependent on precision and accuracy of composition data used in mine planning and actual execution of each mining bench. Thus, accurate sampling is important because the sample collected is analyzed for composition.
Current sampling techniques, both those that extract the sample from the stream of drill cuttings during the course of drilling such as with a fixed sample cutter in the stream of cuttings and those that extract the sample after completion of drilling from accumulations of cuttings, cannot assure that a sample taken for analysis is representative of all the material removed from the hole-fines and coarse alike. The desired result is diminished because of size classification effects and loss of fines. This is particularly the case where the sample is taken from a pile of cuttings after drilling because fines are not retained in a static collection of material in a pile or are not distributed uniformly in the pile to be sampled due to windy conditions or other causes of size classification.
Methods used in the past to obtain suitably precise and accurate samples representing blast hole cuttings are recognized as being deficient in providing suitable data for efficient mine production planning or involve sufficient human labor to be considered economically prohibitive. The majority of past methods are generally considered to be statistically invalid. The critical requirement is reliable transfer of cuttings from blast hole drilling into a container where an appropriate reduction procedure has been applied to derive a practical size sample for testing. During the transfer process, only a minimal quantity of representative material from the drill hole should be lost. Losses are typically from the finest portion (dust) contained in drilled out material. Fines subject to loss during sample collection can be as much as twenty per cent (or sometimes greater) of material mass removed from the hole during drilling. Fines typically are substantially different in elemental composition from average material composition of the hole due to selective mineral fractionation during drill action. Therefore, separation and loss of fines potentially introduces significant bias in test results when the proportion of fines taken into the sample differs from the proportion of fines occurring in the drilled out material mass produced during excavation of the drill hole.
Drill hole material sampling should have as its objective minimizing loss of fines from the mass during sampling to assure essentially all fines particles are included in the sample for testing at about the same proportion as fines are present in the mass of material produced during drilling. However, completely representative sampling is unlikely to be accomplished in practice as some degree of fines loss will be experienced under practical operating conditions in carrying out a sample extraction procedure. A practical standard for sampling is to maintain loss offices to less than one percent of the drill hole mass as a reasonably allowed quantity so as to maintain accuracy of sampling results at an acceptable level. The objective of one percent allowable fines loss for a particular application of drill hole sampling may be adjusted to a greater or lesser value according to metallurgical and geophysical characteristics of the ore deposit being sampled. The quantity of sample extracted from drill hole cuttings to implement an efficient system of mine planning is entirely dependent upon metallurgical and geophysical properties of the ore deposit. Calculations of the quantity of sample reserved for testing is performed according to generally accepted practice. The concept may be illustrated by an example, which could be considered typical, where weight of material removed from the earth during a blast hole drilling operation is 500 kg. Based on physical properties of the ore body, a primary sample weight of five per cent, or 25 kg., is to be extracted by means of a primary sampling device. Maximum particle size from drilling is in the range of 25 mm. This quantity, 25 kg., requires reduction in weight, to about 5 kg. in typical cases, for presentation to the laboratory for composition analysis. Maximum particle size is of necessity to be reduced by crushing and grinding to a lesser value, perhaps 5 mm., to maintain sample representation validity in carrying out reduction with a secondary sampler. The final stage of reduction can be carried out in the field with an automatic second-stage reduction system installed with the primary sampler and collection system at the drill rig, or an entire 25 kg. primary sample mass can be transported to the laboratory for reduction. The sample handling procedure is selected according to specific circumstances of the mine operation.